The present invention relates to a terminal device, a method of controlling a terminal device, and a computer-readable recording medium storing a program.
A positioning system has been used in practice which locates the present position of a GPS receiver (hereinafter called “receiver”) utilizing a satellite positioning system (SPS) using a position information satellite such as a global positioning system (GPS).
The receiver estimates the frequency of a signal received from a GPS satellite (hereinafter may be simply called “satellite”) based on GPS satellite orbital information stored in advance and the like, and achieves synchronization with the estimated frequency (hereinafter called “estimated frequency”) to receive the signal from the satellite (hereinafter called “satellite signal”). The receiver achieves synchronization after down-converting the frequency of the satellite signal received through an antenna using a clock signal generated by a local oscillator provided in the receiver.
However, since the local oscillator of the receiver changes in frequency due to a change in temperature (hereinafter called “drift”), the frequency after down conversion (hereinafter called “actual reception frequency”) differs from the estimated frequency if no measures are taken, thereby making it impossible to promptly achieve synchronization.
In order to solve this problem, technology has been proposed in which the receiver stores information indicating the difference in frequency of the receiver during the preceding positioning, and achieves synchronization based on the information indicating the difference in frequency during the preceding positioning when subsequently receiving the signal from the satellite (e.g. JP-A-5-256926 (e.g. FIG. 1)).
The receiver correlates a coarse/acquisition (C/A) code contained in the satellite signal with a C/A code replica held by the receiver in order to receive the satellite signal.
In the correlation process, the receiver performs a coherent process and an incoherent (accumulation) process.
The receiver performs the correlation process at specific frequency steps within a specific frequency band around the estimated frequency (frequency at each frequency step is hereinafter called “search frequency”).
The receiver fixes the search frequency of the receiver within the accumulation time. However, the drift also occurs within the accumulation time.
Therefore, the frequency may differ from the actual reception frequency due to the drift within the accumulation time, whereby synchronization with the satellite signal may not be achieved.
In particular, since it is difficult to distinguish the satellite signal from noise under a weak electric field (e.g. indoors) in which the field intensity of the satellite signal is weak, it is necessary to increase the accumulation time (incoherent time) of the correlation process in order to receive the satellite signal while distinguishing the satellite signal from noise.
However, since the drift increases along with an increase in the accumulation time, the actual reception frequency differs from the search frequency to a large extent, whereby the satellite signal cannot be efficiently received.